General Purpose 38mm submin

[Neutronium95]

After snagging some J510s on sale at the end of 2022, I've been wanting to use one to take a crack at the J record. There are also a bunch of other fun motors in 38mm that I think will be fun to fly in submin configurations, including but not limited to the J1026, K627, I1299, I33.

So I decided to build a nosecone and avbay and then attach it to the top of a bunch of different motors. I'll probably build a few different fin cans, and in the case of the composite cased motors I'll bond fins right to the motor.

So far I've done some preliminary CAD and Openrocket work, just to confirm that things will fit where they're supposed to. My first headache is that Aerotech decided to put a 5/16 thread in their 38mm threaded forward closures, and that's just a smidge too wide to fit the electronics around. If it was 1/4" they would fit fine, but instead I've needed to come up with a mildly cursed workaround to adapt it down to 10-32. It'll work, but I really wish that Aerotech would have just made the 38 and 54mm threaded closures with 1/4-20 threads in the first place.

The nosecone is going to be made in my usual way, with three layers of 1.5" Soller fiberglass sleeve laid up over a 3D printed plug. I'll be making the coupler by laying up the same sleeve in a female mold, and I'll probably just 3D print the avbay and bulkhead.

I'm planning on using some 1/16" thick high modulus carbon plate for the fins, and I'll probably just steal a bit of Wildman fiberglass tube from an unbuilt kit for the fin can. I'm considering seeing if I can get an aluminum leading edge machined for the fin can, but I might just sand a bevel into the fiberglass tube.

Recovery will be with an 18" Top Flight thin mill parachute released by a cable cutter. I'm not sure if I'll use a 9" parachute for a drogue, or if I'll go with something smaller.

I'm also intentionally leaving a bit of performance margin on the table, at least for the first configuration. After dealing with too little volume for the parachutes on multiple previous submin projects, I'm leaving a bit of extra space. This will probably reduce performance to the point where it'll be difficult to claim the J record, even with the advantage of flying at Alamosa. Curt von Delius built on hell of a rocket to claim the current J record. If everything goes well, I might see if I can shrink the nose a bit, maybe reduce the fin area a bit, and stick a J1026 in it next year.

 

[pugachu]

What type of adapter would you need to be ideal with the 5/16-18 on Aerotech's tapped forward closure?

McMaster carr has a lot of options, like 91344A880.

Also, many people on here (myself included) have lathe, and may be able to make something custom.

Let me know if I can help, my lathe is small and basic, but adapters aren't all that hard to make

 

[SolarYellow]

Could you use a 5/16 bolt and just grind or mill flats on the exposed portions where needed to make it have the same clearance as 1/4-20?

 

[mrwalsh85]

What type of adapter would you need to be ideal with the 5/16-18 on Aerotech's tapped forward closure?

McMaster carr has a lot of options, like 91344A880.

Also, many people on here (myself included) have lathe, and may be able to make something custom.

Let me know if I can help, my lathe is small and basic, but adapters aren't all that hard to make

Beat me to it. definitely - adapters are readily available.

I have a lathe too, but buying an adapter and getting it shipped to you overnight is a way better option than waiting for someone to get out to their shop and machine something.

 

[Neutronium95]

This is the solution I came up with:


I found these threaded inserts on McMaster. I just need to remove the preapplied red loctite. Also the longest 10-32 rod end bolt is just barely long enough to work as an eyebolt for this design.

 

[Neutronium95]

Last night I was able to design and print the avbay, along with a few other bits. I'm able to fit a Blue Raven and Featherweight GPS into ~2" of tube. Things are a bit tight on the battery side of things, they'll be mounted to the sides of the avbay with VHB tape, which has worked quite well for me before. I'll also be including a Featherweight magnetic switch, mounted in the same way.





If possible, I'm going to try to fit a 250 mAh battery in there for the tracker, but that'll probably be dependent on the wall thickness of the coupler. Since I'm planning on using a female mold without a ton of compression, that'll be something I don't have a ton of control over.

I'm currently printing the nosecone mold, hoping to get it, the coupler, and a few parts for Bare Minimum laid up tonight. I've also found a source for the fins, so this build is on track so far.

 

[tfish]

Your rocket looks a lot like one I was playing around with..a few years ago..


IIRC..2 flights..both with SwampGas..I don't recall the specs on the flights..

Tony

 

[kjhambrick]

@Neutronium95 --

Your Av-Bay is BEAUTIFUL !

I REALLY like your long 10-32 rod end and the threaded inserts to adapt to the 5/16-18 forward closure threads.

Aside from the Loc-Tite, have you come up with a way to prevent the threaded insert and the rod end from unscrewing during drogue descent ?

I am also curious about the orientation of your Blue Raven in your Av-Bay and how you plan to wire your deployment charges ?

THANKS !

-- kjh

EDIT: oops - 5/8-18 changed to 5/16-18

 

[Neutronium95]

I initially wanted to have this ready to fly at NSL, but that's definitely not happening, so now I'm aiming for Mudrock.

I laid up a nosecone and coupler that fit nicely with just a bit of sanding. I also got some 1/16" high modulus carbon fiber fins thanks to @charrington.

This design probably won't be capable of breaking the J record, but it should serve as a good first step. I have four J510s, so it's not a waste to fly one in something that isn't optimized to the bleeding edge.

 

[Adrian A]

I'm planning on using some 1/16" thick high modulus carbon plate for the fins,

I started with that last year and to fix the flutter problem I added uni carbon on the fins and over the fillets to get to about 100 mils at the thickest part. It’s mostly in the middle section of the chord, to make it a little closer to a biconic profile.